1. Field of the Invention
This invention relates to a dielectrically-loaded antenna and, primarily, to a quadrifilar helical antenna with a cylindrical dielectric core and an impedance matching structure.
2. Discussion of the Related Art
Dielectrically-loaded antennas and methods for their manufacture are disclosed in the applicant's U.S. Pat. Nos. 5,854,608, 5,945,963, 5,859,621, 6,369,776, 6,690,336, 6,552,693, 6,300,917, 6,886,237, 6,914,580, as well as pending U.S. application Ser. Nos. 09/517,782, 10/987,311, 11/060,215, 11/088,247, 11/472,586 and 11/472,587. The entire contents of these patents and applications are hereby expressly incorporated herein by reference for all purposes.
U.S. Pat. Nos. 5,854,608 and 5,859,621 disclose quadrifilar dielectrically-loaded antennas for operation at frequencies in excess of 200 MHz. Each antenna has two pairs of diametrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core made of a material having a relative dielectric constant greater than 5. The material of the core occupies the major part of the volume defined by the core outer surface. Extending through the core from one end face to an opposite end face is an axial bore containing a coaxial feed structure comprising an inner conductor surrounded by a shielded conductor. At one end of the core the feed structure conductors are connected to respective antenna elements which have associated connection portions adjacent the end of the bore. At the other end of the bore, the shield conductor is connected to a conductor which links the antenna elements and, in these examples, is in the form of a conductive sleeve encircling part of the core to form a balun. Each of the antenna elements terminates on a rim of the sleeve and each follows a respective helical path from its connection to the feed structure.
U.S. Pat. No. 6,369,776 discloses such an antenna in which the shield conductor is spaced from the wall of the bore, preferably by a tube or sleeve of material (preferably plastics) having a relative dielectric constant which is less than half of the relative dielectric constant of the solid material of the core.
Dielectrically-loaded loop antennas having a similar feed structure and balun arrangement are disclosed in U.S. Pat. Nos. 5,954,963, 6,690,336 and 6,300,917. Each of the above antennas has the common characteristic of metallised conductor elements which are disposed about the core and which are top-fed from a feed structure passing through the core. The conductor elements define an interior volume occupied by the core and all surfaces of the core have metallised conductor elements. The balun provides common-mode isolation of the antenna elements from apparatus connected to the feeder structure, making the antenna especially suitable for small handheld devices. One of the objectives in the design of the antennas disclosed in the prior patents is to achieve as near as possible a balanced source or load for the antenna elements. Although the balun sleeve generally serves to achieve such balance, some reactive imbalance may occur owing to constraints on the characteristic impedance of the coaxial feeder structure and on its length. Additional contributing factors are the difference in length between the inner and outer conductors of the feed structure, e.g., as a result of the bent-over part of the inner conductor, and the inherent asymmetry of a coaxial feed. Where necessary, a compensating reactive matching network in the form of a shorted stub has been connected to the inner conductor adjacent the bottom end face of the core, either as part of the device to which the antenna is connected or as a small shielded printed circuit board assembly attached to the bottom end face of the core.
U.S. patent application Ser. No. 11/472,587 discloses a compensating reactive matching network incorporated in a multiple layer printed circuit board seated on the top end face of the core, the board having conductive layers and tracks which form capacitive and inductive elements constituting the matching network. A coaxial feed structure passing through the core is connected to conductors on the board, and the board, in turn, is connected to four coextensive helical antenna elements plated on a cylindrical side surface portion of the core.
Taiwanese Patent No. 1238566 discloses a helical antenna with a ceramic substrate, a matching assisting structure being provided on a top face of the substrate and connected between first and second helical loops for impedance matching adjustment.
It is an object of the invention to provide a practical low-cost alternative to prior dielectrically-loaded antennas with impedance matching structures.